I think there’s a few red flags here that built up to rejection. 1. Hanoi appears to be clamping down on DTV applications, it is better to apply in home country. 2. Cooking school is probably the weakest route in. The strongest options are work-cation route, with business or job from a country outside Thailand. 3. Agents are an inherently suspicious pathway (I don’t want to go into more detail - if you know, you know). 4. Prior ED visa suggests you are not really a tourist, and the DTV is a tourist visa. The government gets a lot more money from ED visa than DTV also.

My recommendation would be to apply in your home country or place of residence outside Thailand, by yourself not an agent, and use work-cation route.